In radio transmission, modulation is the process of varying a relatively high frequency carrier waveform with a modulating waveform, representing information or tonal signals. A popular form of modulation is FM (frequency modulation) wherein the frequency of an RF (radio frequency) carrier wave is varied in accordance with the amplitude of a modulating signal. That is, the frequency of the carrier, as a result of the modulation, increases and decreases about a center frequency (the carrier frequency) while its amplitude remains constant.
Different frequency bands are allocated for various uses and users, such as commercial, military and amateur radio entities. In order to ensure that a particular frequency band does not interfere with an adjacent band, the maximum amount of departure from the center frequency of the carrier is strictly regulated. This departure is called the deviation, and the maximum allowable deviation must not be exceeded.
FM apparatus such as receivers, transceivers, cell phones, mobile radio phones and radio communication equipment in general, must be tested and calibrated to ensure proper operation, as well as compliance with regulations regarding maximum deviation. This is typically accomplished with a variety of test and calibration instruments. For many applications, the size and weight of the test instruments far exceed available space, particularly in airborne environments. Additionally, for accurate test results, use is made of a spectrum analyzer. Such spectrum analyzers can be prohibitively expensive and require highly trained operators for proper use. The present invention provides for a relatively low cost and highly accurate FM test instrument which can calibrate, or even be incorporated into, for example, RF radio communication apparatus, as well as various RF signal generators, modulation analyzers and other test equipment.
The present invention can be incorporated within the Acoustic Sensor Signal Generator (ASSG) aboard the U.S. Navy""s P-3 aircraft to provide a deviation calibrated ASSG. With the present invention the ASSG will be able to provide calibration tones to the sonobuoy receiver and tape recorder before each data collection mission.
Current modulation analyzers are not capable of measuring deviation below a modulation frequency of 10 Hz, yet current data collection systems operate down to 1 Hz. By utilizing the present invention, modulation can be accurately generated and measured well below a modulation frequency of 1 Hz.
Automatic frequency deviation correction apparatus in accordance with the present invention includes a source of RF carrier signal of center frequency Fc and a source of modulating signal of frequency Fm, having positive and negative half cycles and coupled to modulate the frequency of said carrier signal about said center frequency, as a function of the amplitude of said modulating signal, to produce a frequency modulated output signal.
A feedback arrangement is provided which connects said frequency modulated output signal to a gate circuit as a feedback signal. A gate select means for sensing said positive and negative half cycles of said modulating signal is provided and is operable to cause said gate circuit to provide said feedback signal to a first counter, which counts the number of cycles in said feedback signal during said positive half cycle of said modulating signal. The gate select means is additionally operable to cause said gate circuit to provide said feedback signal to a second counter, which counts the number of cycles in said feedback signal during said negative half cycle of said modulating signal, said number of cycles providing an indication of the deviation of said frequency modulated output signal from said center frequency.
A microprocessor connected to said first and second counters is operable to obtain the count difference between said first and second counters, and from said count difference, determine the deviation of said frequency modulated output signal from said center frequency. The microprocessor may be further operable to calculate and generate a correction signal if the difference between a predetermined desired deviation and said deviation determined by said microprocessor is greater than a predetermined value. The microprocessor applies said correction signal to vary the amplitude of said modulating signal until said difference does not exceed said predetermined value.
In one embodiment, an external source may be selected to provide the modulating signal, after the microprocessor has made any necessary corrections to the amplitude of the modulating signal of frequency Fm.